Fabrication method for unoriented phase-I PVF2

ABSTRACT

Fabrication of unoriented phase I crystalline PVF 2  is described wherein commercially available PVF 2  phase II (crystalline form) is placed in a high pressure cell and its temperature is raised slightly over its melting point. The sample is then subjected to abrupt changes in high pressure and the temperature is dropped thereafter. The pressure is then reduced resulting in a product which contains both phase I and phase II forms of PVF 2 . The phase I content of the product varies from a few percent and up depending upon the pressure applied during the quenching step.

BACKGROUND OF THE INVENTION

This invention relates to piezoelectric materials and, moreparticularly, to fabrication of polyvinylidene fluoride (PVF₂) includingunoriented phase I crystallites which has desirable piezoelectriccharacteristics for it to be used in acoustic transducers.

Since the initial discovery of the large piezoelectric response ofpoled, oriented films of PVF₂ by Kawai in 1969, several subsequentstudies have been made to elucidate the mechanisms responsible for thisphenomenon. Since the greatest piezoelectric activity is found for PVF₂films with the phase I crystal structure, it is desirable to find asatisfactory explanation to account for this feature. Mechanisms mostoften discussed are: a bulk polarization of the sample due to fieldinduced reorientation or switching of the molecular dipoles in the polarphase I crystals; non-uniform charge injection leading to an asymmetricdistribution of real charge in the sample; a field induced charge intrapping of injected or ionic charges present as impurities; and sometype of surface phenomenon caused by the strong interaction between thepositive electrode and the film during poling. Regardless of themechanism accounting for the piezoelectric activity of PVF₂ in the phaseI crystal structure, it is desirable to have a simple technique offabricating this material to exploit it for use in acoustic transducers.

SUMMARY OF THE INVENTION

The objects and advantages of the present invention are accomplished byutilizing a method of fabricating PVF₂ in the phase I crystal structureby using a commercially available PVF₂ phase II. The method used toaccomplish this objective is usually termed as pressure quenching(crystallization caused by rapidly increasing pressure) of molten PVF₂.Work samples are cut from the unoriented, melt-pressed, films and theexposed film edges are coated with a thin layer of epoxy. The samplesare then subjected to high pressure. The sample is then melted using asteady-state heating rate. The material is then subjected to a muchhigher pressure in a very short time interval. Immediately after theincrease in pressure, crystallization of the sample is observed. Thesample is then cooled and the pressure is reduced to atmosphericpressure. By varying initial pressure and final pressure, varioussamples are obtained which contained various percentages of PVF₂ phase Iand PVF₂ phase II crystalline material. The samples are then poled usinga poling field below the threshold value so as to produce no observablechanges in the x-ray diffraction patterns taken before and after poling.After poling the sample, piezoelectric constants are measured and thecontribution from PVF₂ phase I crystalline structure is studied.

An object of subject invention is to have a technique of fabricatingPVF₂ phase I crystalline structure.

Another object of the subject invention is to fabricate PVF₂ phase Icrystalline structure from commercially available PVF₂ phase IIcrystalline structure.

Still another object of the subject invention is to have PVF₂ phase Icrystalline structure for acoustic transducer use.

Another object of the subject invention is to obtain optimum parametersunder which PVF₂ phase II samples can be transformed into PVF₂ phase Icrystalline structure.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawing wherein:

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a graphical representation of wide-angle diffractometer scansof samples containing different percentages of phase I crystalline PVF₂material;

FIG. 2 is a graphical representation of variation of piezoelectricstress constant (d₃₁ *) as a function percentage of PVF₂ phase Icrystalline structure in the sample; and

FIG. 3 is a graphical representation of piezoelectric strain constante₃₁ *) as a function of percentage of PVF₂ phase I crystalline materialpresent in the sample.

DESCRIPTION OF A PREFERRED EMBODIMENT

The method of fabricating PVF₂ phase I crystalline structure accordingto the teachings of subject invention involved preparing work samplesfrom 25 micron (25×10⁻⁶ meters), capacitor grade, Kureha film of PVF₂.It should be noted that any equivalent capacitor grade of PVF₂ can alsobe used without deviating from the teachings of subject invention. Toremove the biaxial orientation of the as-received film, it wassandwiched between aluminum foil sheets and melted in a press at 210° C.and kept at 130° C. for 15 minutes thereafter. The stress applied to thesamples by a press was approximately 2000 psi (pounds per square inch).However, 25 micron steel shim stock was placed between the pressureplates to maintain the original thickness of the sample. Actualthickness of the samples varied from 23 to 25 microns. Wide-anglediffractometer scans, taken in both reflection and transmission modes,of samples prepared in this manner showed no evidence of crystalliteorientation.

For pressure quenching thin strips, preferably of 1 cm width, the filmsandwiched in aluminum sheets were cut from the unoriented melt-pressedfilms and the exposed film edges were coated with a thin layer of epoxy.After allowing the epoxy to set, the samples were placed inside the highpressure differential thermal analysis (DTA) cell. The pressure medium,preferably silicon oil, was added and some of the sample was melted ontothe sample thermocouple junction, and the cell was sealed. The aluminumand epoxy surrounding the film samples effectively prevented diffusionof the pressure medium (silicon oil) into the samples and thus had anegligible effect in terms of altering the state of hydrostatic stress(pressure) to which the samples were subjected. The high-pressure systemwas then pumped up to some predetermined initial pressure, P_(i) and theDTA cell was heated externally. The steady-state heating rate,determined from the sample thermocouple was approximately 8° C. perminute. Just before sample melting occurred, the DTA pressure vessel wasisolated from the rest of the high-pressure system by closing a valve inthe high pressure line between the DTA cell and the pressureintensifier. The high-pressure system (intensifier) was then pumped upto some much higher pressure. When the sample had clearly melted, asindicated by the DTA thermogram, the valve was quickly opened and thepressure in the DTA cell rapidly (i.e. less than 0.1 second) wasincreased to a final pressure P_(f). Immediately after the increase inpressure from P_(i) to P_(f) in the DTA cell, sample crystallizationoccurred as indicated by the DTA thermogram. The DTA cell was thencooled to approximately 40° C. by replacing the external heaters withwater cooled copper coils and the pressure was reduced to atmosphericpressure and the samples were then removed.

A measure of the volume fraction of phase I and phase II crystallitespresent in the pressure-quenched samples was determined from thewide-angle diffractometer scans using CuKα radiation. As shown in FIG.1, x-ray intensity was plotted against the values of refracted angle inwide-angle diffractometer scans. Curves 10, 12, 14, 16 and 18 showdifferent diffractometer scans of samples containing differentpercentages in phase I PVF₂ crystalline material. Peaks 20, 22, 24correspond to phase II of PVF₂ crystalline material whereas peaks 30,32, 34 and 36 correspond to phase I PVF₂ crystallite material. As can beseen, the peaks due to phase II PVF₂ crystalline material start fadingand peaks due to phase I PVF₂ crystalline material start appearing withthe increase in percentage of phase I PVF₂ crystalline material and thedecrease in the percentage of phase II PVF₂ crystalline material. Thesamples so obtained were prepared for poling by evaporating goldelectrodes on both sides. The samples were then placed in the polingapparatus which included two polished copper plates connected to a highvoltage supply. The apparatus was then placed under high vacuum and thechamber pressure was reduced to approximately 10⁻⁵ Torr and the sampleswere poled for preferably one hour at 10⁶ volts/cm with polingtemperatures between 21° C. and 23° C. The poling field was chosen to bebelow the threshold value of 1.2×10⁶ volts per centimeter at whichsignificant changes in wide-angle diffractometer scans were observedafter poling. Thus the poling field was kept at 10⁶ volts/cm to avoidthe difficulty of determining the effects of field-induced changes incrystallite orientation and polymorphic crystal form. No measurablechange in sample crystallinity during poling was observed. Dynamicpiezoelectric strain constant (d₃₁ * having a real component d₃₁ ' andimaginary component d₃₁ ") and stress constant (e₃₁ * having a realcomponent e₃₁ ' and an imaginary component e₃₁ ") measurements anddynamic mechanical modulus and dielectric measurements were made at 3hertz (H_(z)) using a Toyo Seiki, Piezotron U, Dynamic Piezo-ElectricityAnalyzer. The results of these measurements are shown in FIGS. 2 and 3.As can be seen, the value of piezoelectric stress constant (e₃₁ *) andpiezoelectric strain constant (d₃₁ *) increase appreciably with theincrease in percentage of phase I PVF₂ crystalline structure.

The initial pressure P_(i) and final pressure P_(f) were varied in orderto determine phase I content (percentage) crystallinity andcrystallization pressures shown on the following page in Table 1.

From FIGS. 1 through 3, it can be seen that phase I PVF₂ crystallinestructure is obtainable by using the technique of subject invention,i.e. pressure quenching of molten PVF₂.

                  TABLE 1                                                         ______________________________________                                        Phase I content (%), % Crystallinity, and Crystallization                     Pressures of the Samples.                                                                   Percent                                                         Phase I Content (%)                                                                         Crystallinity                                                                              P.sub.i  P.sub.f                                   ______________________________________                                        0             53           1 atm.   1 atm.                                    37            51           1.6 Kb   2.7 Kb                                    47            54           1.6 Kb   6.4 Kb                                    64            52           1.7 Kb   6.5 Kb                                    94            51           2.0 Kb   6.6 Kb                                    ______________________________________                                    

Briefly stated, phase I PVF₂ crystalline structure is obtained by usingpressure quenching of molten phase II PVF₂. According to this method,samples of capacitor grade PVF₂ are subjected to sudden pressurevariations in order to obtain varying percentages of phase I PVF₂crystalline structure in the samples.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. As an example,different types of PVF₂ samples including oriented film, unoriented filmand bulk form can be used. Furthermore, pressure and temperature andheating and cooling rates can be changed to optimize conditions toobtain a high percentage of phase I PVF₂ crystalline structure. It is,therefore, to be understood that within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed.

What is claimed is:
 1. A method for converting a predominantly phase IIPVF₂ sample into PVF₂ phase I crystalline structure which includes thesteps of:placing said PVF₂ sample in a high pressure cell; applying highpressure at least 2 kilobars to said sample in said high pressure cell;heating said sample in said high pressure cell to bring the temperaturethereof close to a melting point of said PVF₂ sample; increasing rapidlythe high pressure applied to said PVF₂ sample to a second predeterminedvalue in a predetermined short time interval; cooling said sample insaid high pressure cell; and reducing pressure applied to said sample toatmospheric pressure.
 2. The method of claim 1 wherein the step ofheating said unoriented PVF₂ sample is at a steady rate of 8° C. perminute.
 3. The method of claim 1 wherein the step of increasing rapidlythe high pressure applied to said unoriented PVF₂ sample includes saidsecond predetermined value of 6.6 kilobars.
 4. The method of claim 3wherein the step of increasing rapidly the high pressure applied to saidunoriented PVF₂ sample to said second predetermined value includes saidpredetermined short time interval of 0.1 second.
 5. The method of claim1 which further includes a step of forming an unoriented melt-pressedfilm from said PVF₂ sample prior to the steps of claim
 1. 6. The methodof claim 5 wherein the step of claim 2 for making an unorientedmelt-pressed film from said PVF₂ sample further includes using a pair ofaluminum foils a fixed distance apart for keeping said unorientedmelt-pressed film of a uniform thickness.
 7. The method of claim 6wherein the step of claim 2 for making an unoriented melt-pressed filmfurther includes applying pressure of 2000 psi to the melted sample ofPVF₂.